Article having a poly(arylene sulfide) coating and method of producing

ABSTRACT

Addition of a siliceous material having a particle size of less than 10 microns to a poly(arylene sulfide) coating formulation yields a coating with improved surface characteristics for use as a release coating in conjunction with high temperature molding, extruding, or material handling equipment.

This application is a division of copending application Ser. No. 369,608filed June 13, 1973, now U.S. Pat. No. 3,894,983.

This invention relates to an improved poly(arylene sulfide) coatingcomposition. In another aspect it relates to a poly(arylene sulfide)coated substrate. In still another aspect it relates to an improvedmethod for the release coating of molding and extruding equipment.

The utility of poly(arylene sulfide) as a coating for metal, ceramics,and other surfaces is well known. Poly(arylene sulfides) are also knownfor their high temperature stability. It has been found that addition ofthe proper quantity of a finely divided siliceous material to apoly(arylene sulfide) coating material results in a coating whichretains the strength and high temperature stability common topoly(arylene sulfide) coatings in general, and exhibits improved surfaceproperties making it desirable for use as a release coating in hightemperature applications.

In addition to providing a stable and durable release coating, thepresent invention provides a means for eliminating die lines or otherminor imperfections in molding or forming equipment. The improvedrelease properties of the coating assist in eliminating resin hangups inproduction machinery thereby reducing maintenance costs as well asreducing the production loss for maintenance and repair time. Theimproved release properties also cut down on machine purge time andmaterial expense between production runs. With respect to themanufacture of synthetic rubber tires, coating of the tire mold with thecomposition of the invention eliminates the need for painting the greentire form with a release material prior to molding, eliminates the needfor periodic cleaning of the tire mold, substantially reduces tiredefects such as folds, almost totally eliminates surface defects incured tires, and improves the visual appearance of the finished product.

It is an object of this invention to provide an improved poly(arylenesulfide) coating material. Another object is to provide an improvedmethod for coating a substrate with poly(arylene sulfide). Still anotherobject is to provide a new and useful method for release coating ofmaterials. Yet another object is to provide a method for coating amaterial with a coating having a glossy surface and uniform thickness.Other objects, aspects, and advantages of the invention will be apparentfrom a study of the specification and the appended claims.

Any normally solid poly(arylene sulfide) can be used in the practice ofthis invention. The term poly(arylene sulfide) is meant to include notonly homopolymers but also arylene sulfide copolymers, terpolymers, andthe like, as well as blends of two or more such polymers. Poly(arylenesulfides) which are particularly suited for use in this invention arethose having inherent viscosities in chloronaphthalene (0.2 gram polymerin 100 cc chloronaphthalene) at 206° C. of at least about 0.08,preferably between about 0.1 and about 0.3, and more preferably betweenabout 0.13 and about 0.23. Examples of suitable polymers are disclosed,for instance, in Edmonds et al. U.S. Pat. No. 3,354,129, Nov. 21, 1967.The presently preferred polymer is poly(phenylene sulfide). Otherexamples of poly(arylene sulfides) are poly(4,4'-biphenylene sulfide);poly(2,4-tolylene sulfide); a copolymer from p-dichlorobenzene,2,4-dichlorotoluene, and sodium sulfide; and blends thereof.

In addition, the poly(arylene sulfide) can be blended with otherpolymers, conventional additives, fillers, extenders, pigments,stabilizers, and the like. Especially preferred fillers are TiO₂, Fe₃O₄, and fluorocarbon polymers such as polytetrafluoroethylene. Up toabout 150 parts by weight, either singly or in combination, of suchfillers may be used per 100 parts by weight of poly(arylene sulfide),about 5 to about 80 parts by weight being presently preferred.

In a presently preferred embodiment of the invention, titanium dioxideis mixed with the poly(arylene sulfide) at a concentration within therange of about 0.5 percent to about 100 percent by weight of titaniumdioxide based on the weight of poly(arylene sulfide). More preferably,the concentration of titanium dioxide will be within the range of about10 to about 50 weight percent based on the weight of poly(arylenesulfide). The preferred embodiment also contains from about 0.5 percentto about 75 percent by weight of a fluorocarbon polymer based on theweight of the poly(arylene sulfide). A preferred fluorocarbon polymer ispolytetrafluoroethylene, and the preferred concentration thereof is inthe range of about 5 to about 35 percent by weight of the poly(arylenesulfide).

In accordance with the invention, the substrate can be any metal, glass,or ceramic material to which poly(arylene sulfide) can be adhered andwhich can withstand the high temperature required to cure thepoly(arylene sulfide) coating. Iron, steel, aluminum, titanium, copper,glass, ceramics, porcelain, and alloys such as nickel steel, bronze,etc., are examples of substrates which can be coated in the mannerherein described. The substrate to be coated can be prepared bydegreasing, cleaning, and drying the surface to be coated. Cleaning canbe accomplished by washing or wiping with trichloroethylene, acetone, orother similar cleaners or solvents, then air drying, wiping the surfacedry, or otherwise drying the surface by any suitable method.

The coating composition of the present invention is prepared bycombining poly(arylene sulfide) with from about 0.3 percent to about 4percent by weight of a siliceous material having a particle size of lessthan 10 microns, preferably less than 5 microns, and more preferablyless than 2 microns, including silica; silicates such as calciumsilicate and magnesium silicate; alumino-silicates including artificialzeolites and clays such as bentonite, illite, montmorillanite,kaolinite, attapulgite, and talc; and combinations of these. It has beenfound that concentrations of siliceous material substantially less than0.3 percent by weight based on poly(arylene sulfide) are generallyinsufficient to provide the highly desirable surface characteristics andthat concentrations substantially in excess of about 4 percent by weightbased on poly(arylene sulfide) have a deleterious effect on the bondingstrength of the resultant coating. A siliceous material concentrationwithin the range of about 0.4 percent to about 2 percent by weight basedon the weight of poly(arylene sulfide) is presently preferred, with aconcentration in the range of about 0.5 to about 1 percent by weightbased on the weight of poly(arylene sulfide) being more preferred.

While the poly(arylene sulfide) coating can be applied to the substratein any manner known to the art, a preferred method is to prepare aslurry containing the poly(arylene sulfide) based coating composition,and then to spray coat the substrate with the slurry. Such a spraycoating method is conducive to application of a coating of uniformthickness having a smooth, unbroken surface.

When a slurry is employed, any liquid can be used as the diluentprovided the liquid is inert to the other components of the slurry andto the substrate, and provided it is readily volatilized prior to orduring the subsequent heating or curing treatment. Suitable liquidsinclude water; light hydrocarbons having 5-8 carbon atoms, such ashexane, isooctane, and pentane; benzene; toluene; petroleum ether;alcohols or glycols having 1-4 carbon atoms, such as ethanol, propanol,i-propanol, ethylene glycol, butanol-1, and propylene glycol; ormixtures thereof. A wetting agent, usually a nonionic material such asalkylphenoxypolyethoxy alcohol, in the proportion of about 0.5 percentto about 1 percent by weight based on water can be used. Other suitablewetting agents are octylphenoxypolyethoxy ethanol, aliphatic polyethers,nonylphenylpolyethylene glycol ether, and trimethylnonylpolyethyleneglycol ether. A mixture of water and propylene glycol is presentlypreferred. Use of a diluent containing 60 volume percent water and 40volume percent propylene glycol has yielded excellent results. A highratio of glycol to water prevents premature volatilization of thediluent so that boundaries can be clearly delineated and oversprayingcan be avoided or minimized by the ready coalescence of boundary areas.

Preparation of a suitable slurry can be accomplished in any manner knownto the art. It has been found that a suitable slurry can be obtained byball milling the poly(arylene sulfide), siliceous material, otheradditives, and diluent for a period of time within the range of about 2to about 48 hours. The concentration of the polymer in the carrierliquid depends upon the particular method of coating application andwhether or not additional solids are employed. As a practical matter,the slurry should not be so dilute that the polymer particles are notclosely enough associated to permit them to fuse together on melting. Onthe other hand, the thickness of the slurry and its ease of applicationwill impose a practical upper limit on the solids concentration.Generally, a solids content within the range of about 10 to about 60weight percent can be used, and best results are obtained with a slurrythat contains about 25 to about 40 weight percent solids based on theweight of the total slurry.

The slurry is preferably sprayed onto the substrate without prior heattreatment of the substrate, but the substrate may be heated to atemperature of at least 400° F., more preferably between about 600° andabout 800° F., prior to application of the coating. If the substrate isheated prior to slurry application, it is preferably maintained at atemperature high enough to fuse the polymer on contact during thecoating operation.

After the coating has been applied, the coating is cured by heating at atemperature in the range of about 500° to about 900° F., in anoxygen-containing atmosphere, such as air, for a time in the range ofabout 5 minutes to about 12 hours or more. The thickness of the coatingcan be increased by additional applications following the same procedureafter each curing. Coating thicknesses of about 0.5 to about 50 mils caneasily be applied. Coatings having thicknesses of about 0.5 to about 30mils are satisfactory for most uses.

EXAMPLE I

A slurry was prepared by combining 3 parts by weight poly(phenylenesulfide) with 1 part by weight titanium dioxide, 20 percent by weight ofpolytetrafluoroethylene based on the weight of poly(phenylene sulfide),and 1 percent by weight of bentonite based on the weight ofpoly(phenylene sulfide), and a liquid comprising 60 volume percent waterand 40 volume percent propylene glycol, in a ratio of 28 parts by weightof solids to 72 parts by weight of liquid. The slurry was then ballmilled for a period of 24 hours. The extruder mandrel and head of a blowmolding machine were spray coated with the slurry to a thickness of 5mils. The coated surfaces were then cured at 700° F. for 2 hours. Acoating of uniform thickness and having a smooth, high gloss finish wasobtained which readily released plastic remainders after operating at425° F. for several hours.

Additional tests using the above procedure but varying the proportion ofbentonite disclosed that a concentration of bentonite of 0.25 percent byweight based on the weight of poly(phenylene sulfide) was generallyineffective in improving the release properties and surfacecharacteristics of the coating and that a concentration of bentonite of5 percent by weight based on the weight of poly(phenylene sulfide) has adeleterious effect on the bonding strength of the coating to thesubstrate.

EXAMPLE II

A 10.00 × 20 truck tire mold was coated with a 1 mil thick releasecoating using a slurry of poly(phenylene sulfide), 25 percent by weightof titanium dioxide based on the weight of poly(phenylene sulfide), and18.8 percent by weight of polytetrafluoroethylene based on the weight ofpoly(phenylene sulfide). The coating composition did not contain any ofthe finely divided siliceous material used in the invention. The coatingwas cured at a temperature of 700° F. for 3 hours. The coated mold wasthen tested under actual operating conditions to determine theeffectiveness and durability of the coating composition. Although thecoated surface of the mold began to show an accumulation of rubberproducts after releasing its first 1000 tires, operation of the mold wascontinued as long as it produced an acceptable product so that somemeasure of the useful life of the coating could be obtained. The moldsuccessfully demonstrated the endurance of the poly(phenylene sulfide)coating by producing several thousand more tires without being cleanedand without producing any tires which were unsuitable by reason of moldsticking. The surface of the tires produced by this mold was consideredto be suitable only for truck, tractor, and heavy equipment tires,however, due to the roughened appearance of the finished surface of thetires.

EXAMPLE III

A slurry was prepared by combining 4 parts by weight poly(phenylenesulfide) with 1 part by weight titanium dioxide, 20 percent by weight ofpolytetrafluoroethylene based on the weight of poly(phenylene sulfide),and 1/2 percent by weight of bentonite based on the weight ofpoly(phenylene sulfide) and a liquid comprising one part by volumewater, one part by volume propylene glycol, and 0.5 percent by weightoctylphenoxypolyethoxy ethanol based on the weight of water, in a ratioof 27 parts by weight of solids to 73 parts by weight of liquid. Theslurry was then ball milled for a period of 48 hours. A 10.00 × 22 tiremold was spray coated with the slurry to apply a 1 mil thick coating tothe mold, and the coated mold was cured at 700° F. for 3 hours. Visualinspection of the coating showed that, compared to similar coatings notcontaining any finely divided siliceous material, the addition ofbentonite caused a decrease in the number and magnitude of surfaceimperfections and resulted in a smooth, high gloss surface. The mold wasthen tested under actual operating conditions to determine theeffectiveness and durability of the coating composition. After producingseveral thousand tires without being cleaned, the mold continued toproduce tires having a high quality surface suitable for passenger cartires, showed no sign of rubber product sticking, and, due to theparticularly good release properties of the coating, accumulated littleor no rubber product on the release surface.

Reasonable variations and modifications are possible within the scope ofthe foregoing disclosure and the appended claims to the invention.

I claim:
 1. A coated article comprising a substrate having a coatingbonded thereto of a composition comprising a normally solid poly(arylenesulfide) and from about 0.3 percent to about 4 percent by weight, basedon the weight of poly(arylene sulfide), of a finely divided siliceousmaterial having a particle size less than about 10 microns.
 2. Thearticle of claim 1 wherein said siliceous material is selected from thegroup consisting of silica, silicates, alumino-silicates, andcombinations thereof.
 3. The article of claim 2 wherein the normallysolid poly(arylene sulfide) is poly(phenylene sulfide).
 4. The articleof claim 2 wherein said composition contains from about 0.5 percent toabout 100 percent by weight titanium dioxide based on the weight ofpoly(arylene sulfide) and from about 0.5 percent to about 75 percent byweight fluorocarbon polymer based on the weight of poly(arylene sulfide.5. The article of claim 4 wherein said fluorocarbon polymer ispolytetrafluoroethylene, wherein said poly(arylene sulfide) ispoly(phenylene sulfide), wherein said composition contains from about 10to about 50 percent by weight titanium dioxide based on the weight ofpoly(phenylene sulfide), from about 5 to about 35 percent by weightpolytetrafluoroethylene based on the weight of poly(phenylene sulfide),and from about 0.4 to about 2 percent by weight of said siliceousmaterial based on the weight of poly(phenylene sulfide), and wherein theparticle size of said siliceous material is less than about 2 microns.6. The article of claim 5 wherein said siliceous material is bentoniteclay.
 7. The method of producing a glossy poly(arylene sulfide) coatingof substantially uniform thickness comprising:covering the surface ofthe article to be coated with a slurry containing poly(arylene sulfide)and from about 0.3 to about 4 weight percent of a finely dividedsiliceous material having a particle size less than about 10 microns,based on the weight of poly(arylene sulfide); and curing thethus-covered surface in oxygen-containing atmosphere at a temperaturewithin the range of about 500° F. to about 900° F. for a period of timewithin the range of about 5 minutes to about 12 hours.
 8. The method ofclaim 7 wherein said siliceous material is selected from the groupconsisting of silica, silicates, alumino-silicates, and combinationsthereof.
 9. The method of claim 8 wherein said poly(arylene sulfide) ispoly(phenylene sulfide).
 10. The method of claim 8 wherein said slurrycomprises, in addition, from about 10 percent to about 50 percent byweight titanium dioxide based on the weight of poly(phenylene sulfide)and from about 5 percent to about 35 percent by weight ofpolytetrafluoroethylene based on the weight of poly(phenylene sulfide),and wherein said siliceous material is present in said slurry in anamount in the range of about 0.5 to about 1 weight percent based on theweight of poly(phenylene sulfide).